(Beyond Pesticides, August 2, 2018) Weed scientists from the University of Missouri (UM) have just published evidence of a water hemp population resistant to six different herbicides. The study is sending shock waves throughout the chemical-intensive agricultural community, particularly in light of the plant’s resistance to 2,4-D. In its reporting on the study, KTIC Rural Radio begged the question, “If we’re already seeing 2,4-D resistance now, what will happen when use of the herbicide becomes even more commonplace?”

KTIC is referring to the impending commercialization of products like Enlist Duo, developed by DowDupont in an attempt to address widespread weed resistance to glyphosate. Enlist Duo is an herbicide containing both glyphosate and 2,4-D, and is intended to be sprayed only on crops genetically engineered (GE) to tolerate exposure to both chemicals. However, with growing reports like this, many farmers may begin to rethink their approach.

In 2014, a farmer contacted UM indicating that water hemp was not responding to 2,4-D during a regular ‘burndown’ the farmer conducted before planting a new crop. (Chemical-intensive farmers will often use a synthetic herbicide to clear their field for new plantings.) The farmer had also used other herbicides, fomesafen and glyphosate, in this process and similarly saw no control. Researchers conducted a series of field and dose-response experiments to determine what chemicals the plants were resistant to and to what degree.

Field data showed that typical 2,4-D application rates controlled between 26% and 77% of water hemp in 2015 but only 15% to 55% in 2016. Based on dose-response experiments researchers determined that the population they were studying was 3 times more resistant to 2,4-D than a susceptible population. For the other herbicides tested, including atrazine, fomesafen, glyphosate, and mesotrione, compared to the susceptible population of water hemp studied, was 7,7,22, and 14 times more resistant, respectively. Researchers were unable to determine the degree of resistance to the herbicide chlorimuron, but the product only provided 7% control of water hemp in field studies.

The only two herbicides the water hemp population was not resistant to were dicamba and glufosinate. These two pesticides happen to be the products that make up Bayer’s Liberty Link GE cropping system. The use of dicamba-based products has resulted in widespread nontarget damage throughout the Midwest, making them a poor alternative choice for farmers.

Researchers note that water hemp is the biggest weed issue in corn and soybean fields throughout the Midwest. It has the ability to produce upwards of 1,000,000 seeds during a growing season, can germinate quickly and early, and competes aggressively with row crops, resulting in significantly reduced yields.

While the pesticide industry argues that resistance can be managed by stacking pesticides, planting GE-free refuges, and other techniques, the fact remains that resistance is a predictable aspect of a system reliant on toxic chemicals to manage pests. Rather than recommend switching to dicamba or another herbicide, researchers are suggesting that farmers adopt a more diversified method of weed management, including the use of cultural, mechanical, and biological approaches.

Beyond Pesticides encourages readers to support farming systems already following these recommendations by purchasing organic whenever possible. Organic agriculture never allows the use of toxic pesticides and GE crops, and requires that farmers have an organic systems plan that includes how they will manage weed and other pest issues if or when they arise. For more information on resistance in GE agriculture, see Beyond Pesticides’ program page.

All unattributed positions and opinions in this piece of those of Beyond Pesticides.